1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
An image reading apparatus such as a scanner has widely been used in a multifunction peripheral which converts image information of a read original into digital data and outputs the digital data to an image forming apparatus such as a printer or a storage device such as a computer connected to a network. Recently, a user need is arising for image reading apparatuses having a high productivity in image reading processing. Further, it is becoming an important specification for image reading apparatuses to have a short FCOT (First Copy Output Time) until the first copy is output after a reading start instruction.
Some conventional image reading apparatuses adopt an arrangement to set an image reading clock for each color mode when reading a monochrome image original, full-color image original, or the like. The image reading clock is set in accordance with the color mode. For example, a high-speed image reading clock is set in the monochrome image mode, realizing high-speed reading and high productivity in the monochrome image mode. For example, according to Japanese Patent Laid-Open No. 8-251355, pre-scanning is executed to determine a color mode. In actual scanning, different image reading clocks are set for the full-color image mode and monochrome image mode.
Image reading apparatuses of another type employ an arrangement not to perform pre-scanning by setting a constant reading speed and image transfer speed in both the monochrome image mode and full-color image mode. An image reading apparatus of this type executes image processing, including color mode determination of which of a monochrome or full-color image is read, extraction of a text, and extraction of image data of a picture. However, the image reading apparatus performs the same processing regardless of whether a read original image is a monochrome or full-color image.
Considering the user need for higher-productivity image reading processing, a specification to execute a high-speed reading operation in the monochrome image mode and a high-quality reading operation in the full-color image mode is examined as another user need. For image reading to meet this user need, there is an image reading apparatus with a line sensor arrangement made up of a total of four lines, for example, one line of a monochrome line sensor and three lines of full-color line sensors (RGB). For example, Japanese Patent Laid-Open No. 2003-274115 discloses setting of clocks in the full-color image mode and monochrome image mode in an image reading apparatus.
According to Japanese Patent Laid-Open No. 8-251355, clocks are set selectively in the full-color image mode and monochrome image mode. This image reading apparatus executes pre-scanning to detect a color mode before actual scanning for reading an image. The image clock is switched between the monochrome image mode and the full-color image mode. In this case, reading control can be optimized for each color mode. However, in terms of shortening the FCOT, it takes time to end reading because of redundant pre-scanning executed to specify a color mode.
According to Japanese Patent Laid-Open No. 2003-274115, the image clock is set in each color mode so that the monochrome line sensor reads an image at high speed in the monochrome image mode and the full-color line sensors read an image at high quality by in the full-color image mode. However, this reference does not mention a method of determining which sensor is used, and a setting timing to use each sensor.
To efficiently use each sensor, an image transfer clock needs to be set appropriately for each sensor. Even a time of about 0.1 sec greatly affects shortening the FCOT. If the image transfer clock is switched and set after designating the start of a job, the time required to switch the image transfer clock becomes non-negligible and may obstruct shortening the FCOT.